It is desirable in the field of micro electronics to have conductive elements both within and between the components. Micro electronics creates a demand for the availability of high-quality components by solution-processing at both the device level and the integrated circuit (IC) level to take advantage of a potentially inexpensive way to “print” components over a large area and also on non-flat and/or non-rigid media.
Materials systems should be designed to have the desired electronic (and optoelectronic) properties, that are solution-processable in appropriate formulations, and can be further integrated into manufacturing schemes with the appropriate solvent and thermal characteristics.
The functions of semiconductor materials systems and their device structure differ widely depending on the intended use. For example light-emitting diodes as disclosed in WO 90/13148, field-effect transistors, photodiodes as disclosed in WO 96/16449, photoconductors as disclosed in U.S. Pat. No. 5,523,555, memories, or others, all have widely differing structures. In light-emitting diode technologies, the semiconductor material must be capable of light emission by electron-hole recombination, for example. In field-effect transistors, the semiconductor material must be capable of field-effect conduction.
However in all cases, there is a need for electrically conductive elements. Electrically conductive elements are required not only at the IC level as interconnect lines and via contacts to wire up and deliver the appropriate power and signals to the various circuit components, but also at the component level as electrode contacts to the devices (for example, as cathode and anode of light emitting diodes and of photodiodes, and as the source, drain and gate electrodes of field-effect transistors, and of tunnel-dielectric-based electrically programmable memory devices). In some cases, it is desirable to have both the circuit interconnects and device electrodes fabricated of essentially the same conductor materials system.
One way to achieve this is photolithographic patterning of metals such as gold, copper and aluminium. This is not practical in many instances in organic device technologies because of cost or integration issues.
An alternative approach is the use of printable metals. Printable gold or silver paints based on suspensions of large metallic gold or silver particles in a polymer binder dissolved in organic solvents are known for a long time. As the organic solvent evaporates, the metallic gold or silver particles come into contact to provide the requisite electrical conductivity. Similarly, conducting graphite pastes of conductive graphite particles suspended in alcohol solvents are also known. One characteristic of these materials systems is the presence of a significant fraction of large particles more than 50 microns across in the formulations. This may not be particularly suited for future applications in organic device technologies. Furthermore, the polymer binder used in the prior art (such as polymethacrylates, polyvinyl alcohols and epoxides) may not be compatible with organic semiconductor technologies. Large particle size means that the fine features required in a high-performance semiconductor device cannot be achieved. The presence of these polymer binder leads to issues with contamination of the semiconductor material itself, and restricts the possibility for multilevel integration because of re-dissolution issues. Finally, such conductive pastes cannot tolerate temperatures above 200° C., which may occur (briefly) during the processing of the organic device and circuits. An example is the electrically conductive Ink 40-3920 marketed by Epoxies Etc. . . . of Rhode Island.
As an alternative, conductive polymers have been proposed for the interconnects and electrodes in organic semiconductor device technologies [F. J. Touwslager, N. P. Willard and D. M. de Leeuw, “I-line lithography of poly(3,4-ethylenedioxythiophene) electrodes and application in all-polymer integrated circuits”, Applied Physics Letter, 81 (2002) pp. 4556-4558]. The best conductivity that can be provided by such materials to date, based on poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) system, is about 1 S cm−1, extendable to 100 S cm−1 by doping with a high-boiling polyhydroxyl plasticizer. Such conductivity is not sufficient for most applications. It is also highly desirable to develop systems that are free from high-boiling polyhydroxyl plasticizers due to potential detrimental impart on device performance.
Gold nanoparticles have been developed for such applications [D. Huang, F. Liao, S. Molesa, D. Redinger and V. Subramanian, “Plastic-compatible low resistance printable gold nanoparticle conductors for flexible electronics”, Journal of the Electrochemical Society, 150 (2003) pp. G412-417, Fuller, S. B., Wilhelm, E. J., Jacobson, J. M. Ink-jet printed nanoparticle microelectromechanical systems. Journal of microelectromechanical systems 11, 54-60 (2002).]. These systems are based on the use of alkylthiol protected gold clusters and colloids in the 1-5 nanometer-size range. Such metal nanoparticles exhibit a low surface melting temperature of 200-300° C., which is considerably lower than the melting temperature of bulk gold at 1064° C. The material disclosed in this literature is soluble in toluene and other aromatic hydrocarbons, but insoluble in water and the alcohols. The deposition of films and patterns from such aromatic hydrocarbon solvents has a potential to interfere with the integrity of structures formed in a prior step due to swelling and re-dissolution. Furthermore, during deposition and particularly during the thermal annealing necessary to convert it to the conductive material, significant volume shrinkage occurs. The poor cohesion within the film leads to development of micro-cracks. The presence of such defects can be overcome by depositing of thick films, which limit the ultimate critical dimensions that can be achieved. Furthermore, the method does not appear to be capable of producing good adhesion to the substrate since neither the gold nor the alkylthiol monolayer protection can develop the required adhesion strength to pass a standard scotch tape peel test, for example.
It is desirable to not restrict the metal nanoparticles to aromatic hydrocarbon and related solvents. A number of approaches have been reported to prepare metal nanoparticles that are dispersible in polar solvents like water and alcohols. All these approaches are based on a selecting a water-soluble polymer or of a water-soluble molecule as the protection molecule for the nanoparticles. These involve the use of carboxyl and hydroxyl functionalised short chain aromatic and alkane thiols in single phase and two phase brust like processes. One example was by using small aromatic thiols with amine and carboxyl groups like mercaptophenol and using a single phase Brust process [Johnson, S. R., Evans, S. D. and Brydson, R., “Influence of a terminal functionality on the physical properties of surfactant stabilized gold nanoparticles”, Langmuir 14 (1998) pp. 6639-6647]. These were soluble in methanol but were prone to aggregation. With the use of mercaptosuccinic acid [Chen S, and Kimura K., “Synthesis and characterization of carboxylate-modified gold nanoparticle powders dispersible in water”, Langmuir 15 (1999) pp. 1075-1082], a water-dispersible gold cluster system was developed. Unfortunately, the solubility of the metal nanoparticles produced by such approaches appears to be rather limited. In the cited literature, for example, the solubility of 1-nm diameter gold clusters is only 2 mg/mL, and is probably even lower for larger particles. For practical solution processing by printing, a concentration of at least 10 mg/mL (preferably 50 mg/mL) is required owing to the high mass density of these materials. Furthermore, the materials produced in the cited literature also appear to be very sensitive to aggregation, whereby the gold nanoparticles aggregate to give particles larger than 500 nm within a few days after preparation.